zram: update documentation

Several documentation changes.

First, we never mentioned any errors and never required any
error checks in documentation.

Second, mention that there is another way to configure and
manage zram devices -- zramctl, provided by util-linux.

Third, add a bit of clarification on why `mem_used_max' attr
is RW and correct some typos.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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